1. Field of the Invention
The present invention relates to a thermal head that has plural heat generating elements arrayed therein in a main scanning direction and causes, while conveying a recording medium in a sub-scanning direction, the respective heat generating elements to generate heat to record an image and the like on a recording medium and a method of controlling the thermal head, and, more particularly to a technique adapted to obtain a high recording quality with high density.
2. Description of the Related Art
There is known a thermal printer including a thermal head that has plural heat elements (heat generating elements) arrayed therein and a platen roller provided to be opposed to the thermal head. In such a thermal printer, the thermal head is pressed against a recording medium (a recording sheet, etc.), which is conveyed onto the platen roller, via an ink ribbon to record an image and the like. When a thermosensitive recording medium is used, the ink ribbon is unnecessary.
FIG. 9 is a schematic diagram showing a main part of a general thermal printer 10 and is a diagram showing a section in a direction perpendicular to a rotating shaft 31 of a platen roller 30.
The thermal printer 10 shown in FIG. 9 includes a line-type thermal head 20 that has plural heat elements (not shown) arrayed therein in a line shape. A recording sheet 40 is held on the platen roller 30 and moved by the rotation of the platen roller 30.
A general image recorded by the thermal printer 10 has the shape of a horizontally long rectangle. Therefore, depending on a type of the thermal printer 10, a relatively short side (a direction perpendicular to the paper surface in FIG. 9) of the image is set as the length of the thermal head 20 and as the main scanning direction taking into account manufacturing cost and the like. The thermal printer 10 records the image on the recording sheet 40 while conveying the recording sheet 40 (feeding the recording sheet 40 in a right direction on the paper surface in FIG. 9) to form a relatively long side of the image, which is set as the sub-scanning direction.
The thermal head 20 is pressed against the recording sheet 40 via an ink ribbon 50 of a rolled cloth shape rolled between two ribbon cartridges 51. The thermal head 20 has a glaze 21, which is a convex portion standing in the vertical direction and extending in the main scanning direction. Plural heat elements are provided in a line shape along a top surface of the glaze 21. Therefore, during recording, the respective heat elements of the thermal head 20 press the recording sheet 40 with a high linear pressure.
When recording is actually executed, the respective heat elements are caused to generate heat in this state. Then, when the thermal printer 10 is a thermal printer of a sublimation transfer system, dye (thermofusible ink) of the ink ribbon 50 is transferred onto the recording sheet 40 in proportion to thermal energy generated by the heat elements. When the thermal printer 10 is a thermal printer of a thermofusible transfer system, pigment (thermofusible ink) of the ink ribbon 50 containing wax as a binder melts with thermal energy generated by the heat elements and adheres to be transferred on to the recording sheet 40. Therefore, one point of the thermofusible ink transferred onto the recording sheet 40 by the heat elements is formed as one dot.
To form a two-dimensional image with such a thermal head 20 of the line type, it is necessary to move the thermal head 20 and the recording sheet 40 relatively to each other. In other words, the thermal printer 10 sequentially forms dots while feeding the recording sheet 40 in the sub-scanning direction. Then, plural dots are arranged in the sub-scanning direction and changed to be continuous sets of dots one after another and a dot line is formed. Moreover, a plurality of the dot lines are formed in the main scanning direction by the plural heat elements arrayed in the main scanning direction. As a result, a two-dimensional image can be formed over the entire recording sheet 40.
As described above, the thermal printer 10 shown in FIG. 9 records an image on the recording sheet 40 by causing the respective heat elements to generate heat while feeding the recording sheet 40 in the sub-scanning direction using the thermal head 20 of the line type that has the plural heat elements arrayed therein in the main scanning direction. The resolution (the density of the dot line) of the thermal printer 10 depends on the number of heat elements arrayed in the main scanning direction of the thermal head 20.
FIG. 10 is a plan view showing a thermal head 200 in the past.
As shown in FIG. 10, in the thermal head 200, plural heat elements h (h1, h2, h3, h4, h5, h6, etc.) are arrayed in one row in the main scanning direction. A total number of the heat elements h is 2560. Therefore, the thermal head 200 can form 2560 dots per one line in the main scanning direction of the respective heat elements h. Since the resolution of the thermal head 200 is 300 DPI (dots per inch), the heat elements h are arranged side by side over 2560 dots/300 DPI=8.53 inches (216 mm).
FIG. 11 is a block diagram showing a method of controlling the thermal head 200 in the past shown in FIG. 10.
As shown in FIG. 11, in the thermal head 200 in the past, when data of an image that should be formed is inputted, heat history correction is applied to data for the heat elements h. Subsequently, the data for the heat elements h is modulated for driving of the heat elements h by PWM modulation. Dots are formed by the driving of the heat elements h based on the modulated data. An overall image is formed by sets of the dots.
In recent years, the thermal printer 10 (see FIG. 9) is demanded to form an image with high definition and, at the same time, at higher speed. For example, high recording speed equal to or less than 1 microsecond per one dot is demanded of the thermal printer 10. Such improvement of recording speed, which should be called as “ultrahigh speed recording”, causes a temperature rise in the thermal head 200 (see FIG. 10).
The thermal head 200, which is originally a consumable product, is deteriorated more rapidly than usual because of an excessive temperature rise in the thermal head 200 (see FIG. 10) and the durable life of the thermal head 200 is extremely shortened. When the heat elements h (see FIG. 10) are arrayed at high density to form an image with high definition, a heat radiation property of the thermal head 200 is spoiled. As a result, a trailing track is formed regardless of the finish of recording, i.e., a so-called “tailing” occurs, because of the heat stored in the thermal head 200 and a recording quality falls.
To cope with such a problem, for example, there is known a technique for arranging the heat elements h (see FIG. 10), which are arranged in one row, in two rows and using one of the rows for preheating of the recording sheet 40 (see FIG. 9) and the ink ribbon 50 (see FIG. 9) or forming dot lines, which are sets of plural dots arranged in the sub-scanning direction, in two rows to thereby preventing an excessive temperature rise in the respective heat elements h.
For example, JP-A-2006-205520 (hereinafter, Patent Document 1) discloses a thermal head including plural printing dots arranged in a line shape and an electrode layer that supplies a current to the plural printing dots, wherein the thermal heads has a large-area heat element, which is thin and long in a traveling direction, on an entrance side in the traveling direction and has a small-area heat element on an exit side and, in the respective printing dots, plural heat generation areas having different heat generation peak temperatures are formed in a current supply direction when the current is supplied from the electrode layer.
JP-A-2002-370398 (hereinafter, Patent Document 2) discloses a thermal head in which plural heat elements are set in parallel to a direction in which thermal recording paper travels, a mechanism for controlling a temperature history (profile) of thermal energy is provided, and the respective heat elements are independently applied, whereby necessary energy can be supplied by a necessary amount.